An evacuation apparatus is widely used for evacuating a process gas that has been supplied to a vacuum chamber of a substrate processing apparatus. In this substrate processing apparatus such as a CVD apparatus or an etching apparatus, it is required to evacuate the process gas from the vacuum chamber to produce a constant vacuum state therein, and a plurality of vacuum pumps connected in series are used to achieve a desired pumping speed and a desired ultimate pressure.
The above-mentioned evacuation apparatus comprises a booster pump connected to the vacuum chamber, and a main pump connected to the booster pump. Both the booster pump and the main pump are a positive-displacement vacuum pump having a pair of pump rotors disposed in a rotor casing. In this type of vacuum pump, a small gap is formed between the pump rotors and also between the pump rotors and an inner surface of the rotor casing, so that these pump rotors can be rotated in the rotor casing in a non-contact manner.
Generally, a single-stage vacuum pump having a pair of Roots-type single-stage pump rotors is used as the booster pump. This is because the conventional CVD apparatus and etching apparatus do not require a large quantity of the process gas for processing a substrate and thus a quantity of the process gas to be evacuated is not so large.
However, as a substrate such as a semiconductor wafer or a liquid crystal panel, which is an object to be processed, becomes large in size, there has been an increasing need for evacuating a large quantity of the process gas. In order to evacuate the process gas in large quantity, it is required to use large-sized pump rotors or to increase a rotational speed of the pump rotors so as to increase a pumping speed. However, with such a structure, a motor for driving the booster pump is overloaded, resulting in increased power consumption. In addition, the pump rotors are likely to expand due to heat of compression of the process gas and heat generated by the motor, and hence the pump rotors may be brought into contact with the inner surface of the rotor casing, resulting in operation failure. Accordingly, it is difficult for the conventional evacuation apparatus having the single-stage booster pump to evacuate a large quantity of the process gas while maintaining a vacuum state in the vacuum chamber. Under such circumstances, there has been a need for an evacuation apparatus which can evacuate a large quantity of a gas (e.g., a process gas) and can prevent a motor thereof from being overloaded.